Method for producing water-absorbent polymer particles

ABSTRACT

A process for producing water-absorbing polymer particles, wherein a product stream comprising water-absorbing polymer particles is tested for nonferritic metallic impurities by means of an eddy current detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase of International Application No.PCT/EP2008/067049, filed Dec. 9, 2008, which claims the benefit ofEuropean Patent Application No. 0712380.5, filed Dec. 14, 2007.

The present invention relates to a process for producing water-absorbingpolymer particles, wherein a product stream comprising water-absorbingpolymer particles is tested for nonferritic metallic impurities by meansof an eddy current detector.

The production of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

Water-absorbing polymers are used to produce diapers, tampons, sanitarynapkins and other hygiene articles, but also as water-retaining agentsin market gardening.

EP 1 422 257 A1 describes a process for removing impurities from aproduct stream comprising water-absorbing polymer particles by means ofa magnetic separator. According to the description, it is also possibleto remove austenitic steels, such as SUS304 or SUS316. The cause of thisis presumably the known residual magnetism which arises through coldworking.

However, the deposition rates achievable with the magnetic separator,especially in the case of use of plant parts made of relativelyhigh-value austenitic steels, are insufficient.

Metal separators based on the eddy current principle are used, forexample, in wood processing. The underlying principle of eddy currentseparation has already been described in U.S. Pat. No. 400,317.

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles, especially a higherdegree of deposition of impurities.

The object was achieved by a process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) at least one crosslinker,-   c) optionally one or more ethylenically and/or allylically    unsaturated monomers copolymerizable with the monomers specified    under a) and-   d) optionally one or more water-soluble polymers,    wherein the plant parts in contact with product consist at least    partly of nonferritic metallic materials, wherein a product stream    comprising water-absorbing polymer particles is tested for    nonferritic metallic impurities by means of an eddy current    detector.

An eddy current detector generates an alternating magnetic field, whichbuilds up a magnetic field in metals which is opposed to the magneticfield generated by the eddy current detector, which measurably changesthe original alternating magnetic field.

The eddy current detector can be calibrated by means of a test bodycomposed of a nonferritic metallic material, for example a stainlesssteel ball with a diameter of 3.5 mm.

The product flow line is not subject to any restrictions. Suitableproduct flow lines are, for example, pipelines in which thewater-absorbing polymer particles are conveyed pneumatically orgravimetrically. The diameter of the product flow line is preferablyfrom 5 to 50 cm, more preferably from 15 to 40 cm, most preferably from20 to 35 cm.

The trend toward more expensive but also higher-value austeniticmaterials leads to the fact that metallic attritus has an ever lowerresidual magnetism B_(r). By means of customary magnetic separators,these impurities can be separated out only inadequately.

The process according to the invention is therefore particularlyadvantageous when the plant parts in contact with product and hence thenonferritic metallic impurities of the product stream which arisethrough wear have a residual magnetism B_(r) of preferably less than0.05 T, more preferably of less than 0.03 T, most preferably of lessthan 0.02 T.

Suitable materials for the plant parts in contact with product areaustenitic steels with, for example, at least 0.08% by weight of carbon.Advantageously, the austenitic steels comprise, as well as iron, carbon,chromium, nickel and optionally molybdenum, also further alloyconstituents, preferably niobium or titanium.

The preferred materials are materials with materials number 1.45xx toDIN EN 10020, where xx may be a natural number between 0 and 99.Particularly preferred materials are the steels with materials numbers1.4541 and 1.4571, especially steel with materials number 1.4541.

The adaptor piece for the eddy current detector in the product flow lineis of course made from a nonmetallic material, preferably ceramic.

In the process according to the invention, the eddy current detector canbe used at various sites. However, it is particularly advantageouslyused immediately upstream of the product silo or the dispensing station.

The nonferritic metallic impurities detected can be discharged, forexample by means of a deflector present in the product line, into anoff-spec silo.

Of course, the process according to the invention also enables thedetection and discharge of ferritic metallic impurities.

The water-absorbing polymer particles are prepared by polymerization ofa monomer solution or suspension.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 50 g/100 g of water. Ideally, the monomers a)are miscible with water in any ratio.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid. Particularly preferred monomers areacrylic acid and methacrylic acid. Very particular preference is givento acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

The content of acrylic acid and/or salts thereof in the total amount ofmonomers a) is preferably at least 50 mol %, more preferably at least 90mol %, most preferably at least 95 mol %.

The monomers a), especially acrylic acid, comprise preferably up to0.025% by weight of a hydroquinone monoether. Preferred hydroquinonemonoethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol refers to compounds of the following formula

where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogenor methyl and R⁴ is hydrogen or an acyl radical having from 1 to 20carbon atoms.

Preferred R⁴ radicals are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically tolerable carboxylic acids. The carboxylic acidsmay be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹=R²=R³=methyl,especially racemic alpha-tocopherol. R¹ is more preferably hydrogen oracetyl. Especially preferred is RRR-alpha-tocopherol.

The monomer solution comprises preferably not more than 130 ppm byweight, more preferably not more than 70 ppm by weight, preferably notless than 10 ppm by weight, more preferably not less than 30 ppm byweight and especially about 50 ppm by weight of hydroquinone monoether,based in each case on acrylic acid, with acrylic acid salts beingcounted as acrylic acid. For example, the monomer solution can beprepared using acrylic acid having an appropriate hydroquinone monoethercontent.

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized by a free-radicalmechanism into the polymer network. Suitable crosslinkers b) are, forexample, ethylene glycol dimethacrylate, diethylene glycol diacrylate,polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropanetriacrylate, triallylamine, tetraallylammonium chloride,tetraallyloxyethane, as described in EP 530 438 A1, di- andtriacrylates, as described in EP 547 847 A1, EP 559 476 A1, EP 632 068A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301A1 and in DE 103 31 450 A1, mixed acrylates which, as well as acrylategroups, comprise further ethylenically unsaturated groups, as describedin DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, asdescribed, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO90/15830 A1 and WO 2002/32962 A2.

Suitable crosslinkers b) are in particular N,N′-methylenebisacrylamideand N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate, forexample butanediol diacrylate, butanediol dimethacrylate, ethyleneglycol diacrylate or ethylene glycol dimethacrylate, and alsotrimethylolpropane triacrylate and allyl compounds such as allyl(meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters,tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allylesters of phosphoric acid and vinyl-phosphonic acid derivatives, asdescribed, for example, in EP 343 427 A2. Further suitable crosslinkersb) are pentaerythritol diallyl ether, pentaerythritol triallyl ether andpentaerythritol tetraallyl ether, polyethylene glycol diallyl ether,ethylene glycol diallyl ether, glycerol diallyl ether and glyceroltriallyl ether, polyallyl ethers based on sorbitol, and ethoxylatedvariants thereof. In the process according to the invention, it ispossible to use di(meth)acrylates of polyethylene glycols, thepolyethylene glycol used having a molecular weight between 100 and 1000,for example polyethylene glycol-400 diacrylate.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuplyethoxylated trimethylolpropane, of 3- to 20-tuply ethoxylatedtrimethylolethane, in particular di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane,of 3-tuply propoxylated glycerol or of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixed ethoxylated orpropoxylated glycerol or of 3-tuply mixed ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol or of 15-tuplyethoxylated trimethylolpropane, and also of at least 40-tuplyethoxylated glycerol, of at least 40-tuply ethoxylated trimethylolethaneor of at least 40-tuply ethoxylated trimethylolpropane.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol.

The amount of crosslinker b) is preferably from 0.05 to 1.5% by weight,more preferably from 0.1 to 1% by weight, most preferably from 0.3 to0.6% by weight, based in each case on monomer a).

Examples of ethylenically unsaturated monomers c) which arecopolymerizable with the ethylenically unsaturated, acid-bearingmonomers a) are acrylamide, methacrylamide, crotonamide, hydroxyethylacrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate,dimethylaminoethyl acrylate, dimethylaminopropyl acrylate,diethylaminopropyl acrylate, dimethylaminobutyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,dimethylaminoneopentyl acrylate and dimethylaminoneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols orpolyacrylic acids, preferably polyvinyl alcohol and starch.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen.

Therefore, the monomer solution can be freed of dissolved oxygen beforethe polymerization by inertization, i.e. flowing through with an inertgas, preferably nitrogen. The oxygen content of the monomer solution ispreferably lowered before the polymerization to less than 1 ppm byweight, more preferably to less than 0.5 ppm by weight.

The preparation of a suitable polymer and also further suitablehydrophilic ethylenically unsaturated monomers a) are described in DE199 41 423 A1, EP 686 650 A1, WO 2001/45758 A1 and WO 2003/104300 A1.

Suitable reactors are, for example, kneading reactors or belt reactors.In the kneader, the polymer gel formed in the polymerization of anaqueous monomer solution or suspension is comminuted continuously by,for example, contrarotatory stirrer shafts, as described in WO2001/38402 A1. Polymerization on a belt is described, for example, in DE38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a beltreactor forms a polymer gel, which has to be comminuted in a furtherprocess step, for example in a meat grinder, extruder or kneader.

The acid groups of the resulting hydrogels have typically been partiallyneutralized, preferably to an extent of from 25 to 95 mol %, morepreferably to an extent of from 50 to 80 mol % and even more preferablyto an extent of from 60 to 75 mol %, for which the customaryneutralizing agents can be used, preferably alkali metal hydroxides,alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

Neutralization is preferably carried out at the monomer stage. It isdone typically by mixing in the neutralizing agent as an aqueoussolution, as a melt, or else preferably as a solid material. Forexample, sodium hydroxide having a water content of distinctly below 50%by weight can be present as a waxy mass having a melting point of above23° C. In this case, metering as piece material or melt at elevatedtemperature is possible.

However, it is also possible to carry out neutralization after thepolymerization, at the hydrogel stage. It is also possible to neutralizeup to 40 mol %, preferably from 10 to 30 mol % and more preferably from15 to 25 mol % of the acid groups before the polymerization by adding aportion of the neutralizing agent actually to the monomer solution andsetting the desired final degree of neutralization only after thepolymerization, at the hydrogel stage. When the hydrogel is neutralizedat least partly after the polymerization, the hydrogel is preferablycomminuted mechanically, for example by means of a meat grinder, inwhich case the neutralizing agent can be sprayed, sprinkled or poured onand then carefully mixed in. To this end, the gel mass obtained can berepeatedly ground in a meat grinder for homogenization.

The hydrogel is then preferably dried with a belt dryer until theresidual moisture content is preferably below 15% by weight, morepreferably below 10% by weight, most preferably below 8% by weight, thewater content being determined by EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 230.2-05“Moisture content”. If desired, however, drying can also be carried outusing a fluidized bed dryer or a heated plowshare mixer. To obtainparticularly white products, it is advantageous to dry this gel whileensuring rapid removal of the evaporating water. To this end, the dryertemperature must be optimized, the air feed and removal has to becontrolled, and sufficient venting must be ensured in each case. Thehigher the solids content of the gel, the simpler the drying, by itsnature, and the whiter the product. The solids content of the gel beforethe drying is therefore preferably between 25% and 80% by weight. It isparticularly advantageous to vent the dryer with nitrogen or anothernonoxidizing inert gas. If desired, however, it is also possible simplyjust to lower the partial pressure of the oxygen during the drying inorder to prevent oxidative yellowing processes.

Thereafter, the dried hydrogel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

The mean particle size of the polymer particles removed as the productfraction is preferably at least 200 μm, more preferably from 250 to 600μm, very particularly from 300 to 500 μm. The mean particle size of theproduct fraction may be determined by means of the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP220.2-05 “Particle size distribution”, where the proportions by mass ofthe screen fractions are plotted in cumulated form and the mean particlesize is determined graphically. The mean particle size here is the valueof the mesh size which gives rise to a cumulative 50% by weight.

The proportion of particles with a particle size of preferably at least150 μm, more preferably at least 200 μm, most preferably at least 250μm, is preferably at least 90% by weight, more preferably at least 95%by weight, most preferably at least 98% by weight.

The proportion of particles with a particle size of preferably at most850 μm, more preferably at most 700 μm, most preferably at most 600 μm,is preferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

To further improve the properties, the polymer particles may bepostcrosslinked. Suitable postcrosslinkers are compounds which comprisegroups which can form covalent bonds with at least two carboxylategroups of the polymer particles. Suitable compounds are, for example,alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di-or polyepoxides, as described in EP 83 022 A2, EP 543 303 A1 and EP 937736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1,DE 35 23 617 A1 and EP 450 922 A2, or β-hydroxyalkylamides, as describedin DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

Additionally described as suitable postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidone and its derivatives, suchas 2-hydroxyethyl-2-oxazolidone in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/31482 A1.

In addition, it is also possible to use postcrosslinkers which compriseadditional polymerizable ethylenically unsaturated groups, as describedin DE 37 136 A1.

The amount of postcrosslinker is preferably from 0.001 to 2% by weight,more preferably from 0.02 to 1% by weight, most preferably from 0.05 to0.2% by weight, based in each case on the polymer.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the postcrosslinkersbefore, during or after the postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium.Possible counterions are chloride, bromide, sulfate, hydrogensulfate,carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate and lactate.Aluminum sulfate is preferred. Apart from metal salts, it is alsopossible to use polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, from 0.001 to 1.5%by weight, preferably from 0.005 to 1% by weight, more preferably from0.02 to 0.8% by weight, based in each case on the polymer.

The postcrosslinking is typically performed in such a way that asolution of the postcrosslinker is sprayed onto the hydrogel or the drypolymer particles. After the spraying, the polymer particles coated withthe postcrosslinker are dried thermally, and the postcrosslinkingreaction can take place either before or during the drying.

The spraying of a solution of the postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers, plowshare mixers and paddle mixers. Particular preference isgiven to horizontal mixers such as plowshare mixers and paddle mixers,very particular preference to vertical mixers. Suitable mixers are, forexample, Lödige mixers, Bepex mixers, Nauta mixers, Processall mixersand Schugi mixers.

The postcrosslinkers are typically used as an aqueous solution. Theaddition of nonaqueous solvent can be used to adjust the penetrationdepth of the postcrosslinker into the polymer particles.

The thermal drying is preferably carried out in contact dryers, morepreferably paddle dryers, most preferably disk dryers. Suitable dryersare, for example, Bepex dryers and Nara dryers. Moreover, it is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream dryer, for examplea shelf dryer, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed dryer.

Preferred drying temperatures are in the range from 100 to 250° C.,preferably from 120 to 220° C., more preferably from 130 to 210° C.,most preferably from 150 to 200° C. The preferred residence time at thistemperature in the reaction mixer or dryer is preferably at least 10minutes, more preferably at least 20 minutes, most preferably at least30 minutes, and typically at most 60 minutes.

Subsequently, the postcrosslinked polymer can be classified again.

To further improve the properties, the postcrosslinked polymer particlescan be coated or subsequently moistened. Suitable coatings for improvingthe acquisition behavior and the permeability (SFC) are, for example,inorganic inert substances, such as water-insoluble metal salts, organicpolymers, cationic polymers and di- or polyvalent metal cations.Suitable coatings for dust binding are, for example, polyols. Suitablecoatings against the undesired caking tendency of the polymer particlesare, for example, fumed silica, such as Aerosil® 200, and surfactants,such as Span® 20.

The water-absorbing polymer particles tested for nonferritic metallicimpurities in accordance with the invention have a water content ofpreferably less than 15% by weight, more preferably less than 10% byweight, most preferably less than 8% by weight, the water content beingdetermined by the EDANA (European Disposables and Nonwovens Association)recommended test method No. WSP 230.2-05 “Moisture content”.

The water-absorbing polymer particles tested for nonferritic metallicimpurities in accordance with the invention have a centrifuge retentioncapacity (CRC) of typically at least 15 g/g, preferably at least 20 g/g,preferentially at least 22 g/g, more preferably at least 24 g/g, mostpreferably at least 26 g/g. The centrifuge retention capacity (CRC) ofthe water-absorbing polymer particles is typically less than 60 g/g. Thecentrifuge retention capacity (CRC) is determined by the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP241.2-05 “Centrifuge retention capacity”.

The water-absorbing polymer particles tested for nonferritic metallicimpurities in accordance with the invention have an absorption under apressure of 0.7 psi (4.83 kPa) of typically at least 15 g/g, preferablyat least 20 g/g, preferentially at least 22 g/g, more preferably atleast 24 g/g, most preferably at least 26 g/g. The absorption under apressure of 0.7 psi (AUL0.7 psi) of the water-absorbing polymerparticles is typically less than 35 g/g. The absorption under a pressureof 0.7 psi (AUL0.7 psi) is determined analogously to the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP242.2-05 “Absorption under pressure”, except that a pressure of 0.7 psi(4.83 kPa) instead of a pressure of 0.3 psi (2.07 kPa) is set.

1. A process for producing water-absorbing polymer particles bypolymerizing a monomer solution or suspension comprising a) at least oneethylenically unsaturated monomer which bears acid groups and may be atleast partly neutralized, b) at least one crosslinker, c) optionally oneor more ethylenically and/or allylically unsaturated monomercopolymerizable with the monomer specified under a), and d) optionallyone or more water-soluble polymers, wherein plant parts in contact withproduct comprise at least partly of nonferritic metallic materials,wherein a product stream comprising the water-absorbing polymerparticles is tested for nonferritic metallic impurities by means of aneddy current detector.
 2. The process according to claim 1, wherein thenonferritic metallic materials are austenitic steels.
 3. The processaccording to claim 2, wherein the austenitic steels have a residualmagnetism of at most 0.05 T.
 4. The process according to claim 1,wherein the water-absorbing polymer particles tested have a watercontent of less than 15% by weight.
 5. The process according to claim 1,wherein at least 95% by weight of the water-absorbing polymer particlestested have a particle size of at least 150 μm.
 6. The process accordingto claim 1, wherein at least 95% by weight of the water-absorbingpolymer particles tested have a particle size of at most 850 μm.
 7. Theprocess according to claim 1, wherein the water-absorbing polymerparticles tested have a centrifuge retention capacity of at least 15g/g.
 8. The process according to claim 1, wherein the water-absorbingpolymer particles tested have an absorption under a pressure of 0.7 psi(4.83 kPa) of at least 15 g/g.
 9. The process according to claim 1,wherein nonferritic metallic impurities detected by means of the eddycurrent detector are discharged from the product stream.
 10. The processaccording to claim 1, wherein a product stream comprising nonferriticmetallic impurities is diverted to a separate silo.